sedimentary
Chert (Nodule)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to brown with waxy luster, Structure: Microcrystalline, Breakage: Conchoidal fracture, Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to brown with waxy luster, Structure: Microcrystalline, Breakage: Conchoidal fracture, Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich fluids within limestone or chalk beds, often from the remains of diatoms or radiolarians. Typically dates to various geological eras including the Cretaceous.
Uses & applications
Historically used for stone tools and arrowheads; currently used as construction aggregate, road surfacing, and as a polishing media in lapidary rock tumblers.
Geological facts
Chert is so hard it can create sparks when struck against steel. It was one of the most vital materials for prehistoric humans due to its ability to hold a sharp edge.
Field identification & locations
Identify in the field by its smooth, waxy feel and circular (conchoidal) fractures. Found globally in sedimentary basins. Collectors look for unique colors or fossil inclusions.
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